US 8199804 B1 Abstract Methods and apparatus are provided for performing equalization of communication channels. In an embodiment of the invention, at least one tap can be selected from a set of feedforward taps of feedforward filter circuitry, where each tap of the selected at least one tap has a magnitude that is greater than or substantially equal to a magnitude of any tap of the set of feedforward taps that is not in the selected at least one tap. In addition, at least one tap can be added to a set of taps of feedback filter circuitry in communication with the feedforward filter circuitry. The invention advantageously allows for more efficient and reliable equalization of communication channels.
Claims(37) 1. A method of configuring equalization circuitry, said method comprising:
receiving a first set of feedforward taps with a plurality of data inputs of a multiplexer, wherein respective ones of feedforward taps are coupled to respective ones of the plurality of data inputs, and wherein an output of the multiplexer is coupled to feedforward filter circuitry;
receiving a first set of feedback taps of feedback filter circuitry;
coupling one of the plurality of data inputs of the multiplexer to the output of the multiplexer, based on a control signal received at a control input of the multiplexer, to filter an input signal with the feedforward filter circuitry, wherein the coupling generates a second set of feedforward taps; and
adding, in response to the coupling, a compensating set of feedback taps to said first set of feedback taps to generate a second set of feedback taps in communication with said feedforward filter circuitry to filter said input signal.
2. The method of
computing said first set of feedback taps; and
computing said first set of feedforward taps based on a result of said computing said first set of feedback taps.
3. The method of
said computing said first set of feedback taps comprises performing a feedback Cholesky factorization; and
said computing said first set of feedforward taps comprises performing a feedforward back-substitution.
4. The method of
generating a feedforward output signal from said feedforward filter circuitry;
generating a feedback output signal from said feedback filter circuitry;
computing an error signal based at least in part on said feedforward output signal and said feedback output signal; and
adapting said second set of feedforward taps and said second set of feedback taps based on said computed error signal.
5. The method of
performing at least one least-mean-square algorithm;
altering a value of a tap of said second set of feedforward taps based on a result of said performing said at least one least-mean-square algorithm; and
altering a value of a tap of said second set of feedback taps based on a result of said performing said at least one least-mean-square algorithm.
6. The method of
7. The method of
comparing a magnitude of a lowest-magnitude tap of said second set of feedforward taps to a magnitude of a highest-magnitude tap in said first set of feedforward taps that is not in said second set of feedforward taps wherein:
said magnitude of said lowest-magnitude tap is less than or substantially equal to a magnitude of any other tap of said second set of feedforward taps;
said magnitude of said highest-magnitude tap is greater than or substantially equal to a magnitude of any other tap of said first set of feedforward taps that is not in said second set of feedforward taps;
removing said lowest-magnitude tap from said second set of feedforward taps if said magnitude of said lowest-magnitude tap is less than said magnitude of said highest-magnitude tap; and
adding said highest-magnitude tap to said second set of feedforward taps if said magnitude of said lowest-magnitude tap is less than said magnitude of said highest-magnitude tap.
8. The method of
sorting said second set of feedforward taps by magnitude to form an ordered set of feedforward taps; and
selecting at least one consecutive tap from said ordered set of feedforward taps, wherein each tap of said selected at least one consecutive tap has a magnitude that is greater than or substantially equal to a magnitude of any tap of said ordered set of feedforward taps that is not in said selected at least one consecutive tap.
9. The method of
processing an input signal using said second set of feedforward taps to generate a feedforward output signal;
adding a signal responsive to said feedforward output signal to a signal responsive to a feedback output signal to generate an adder output signal;
comparing a voltage of a signal responsive to said adder output signal to at least one voltage threshold to generate a decision output signal; and
processing a signal responsive to said decision output signal using said second set of feedback taps to generate said feedback output signal.
10. The method of
said processing said input signal using said second set of feedforward taps comprises performing at least one multiplication on said input signal and at least one value of said selected at least one tap; and
said processing said signal responsive to said decision output signal using said second set of feedback taps comprises performing at least one multiplication on said decision output signal and at least one value of said second set of feedback taps.
11. A method of operating a high-definition television comprising the method of
12. A method of operating a set-top box comprising the method of
13. The method of
determining changing channel characteristics and in response to the determining, adapting the second set of feedforward taps and the second set of feedback taps.
14. Circuitry for equalizing an input signal, said circuitry comprising:
a multiplexer operable to receive a first set of feedforward taps with a plurality of data inputs, wherein respective ones of feedforward taps are coupled to respective ones of the plurality of data inputs;
feedforward filter circuitry operable to receive said input signal, wherein said feedforward filter circuitry is coupled to an output of the multiplexer;
feedback filter circuitry in communication with said feedforward filter circuitry, wherein said feedback filter circuitry is configured to select a first set of feedback taps; and
control circuitry in communication with said feedforward filter circuitry and said feedback filter circuitry, said control circuitry comprising:
selector circuitry configured to generate a control signal received at a control input of the multiplexer, based on which one of the plurality of data inputs of the multiplexer is coupled to the output of the multiplexer to filter said input signal, wherein the coupling generates a second set of feedforward taps; and
feedback tap addition circuitry in communication with said feedback filter circuitry, wherein said feedback tap addition circuitry is configured to generate a second set of feedback taps by adding, in response to the coupling, a compensating set of feedback taps to said first set of feedback taps to filter said input signal.
15. The circuitry of
said control circuitry further comprises tap computation circuitry in communication with said feedforward filter circuitry and said feedback filter circuitry; and
said tap computation circuitry comprises:
circuitry configured to compute said first set of feedback taps; and
circuitry configured to compute said first set of feedforward taps based on a result of said computing said first set of feedback taps.
16. The circuitry of
circuitry configured to compute said first set of feedback taps in accordance with a feedback Cholesky factorization; and
circuitry configured to compute said first set of feedforward taps in accordance with a feedforward back-substitution.
17. The circuitry of
first adaptation circuitry in communication with said feedforward filter circuitry, wherein said first adaptation circuitry is configured to adapt said second set of feedforward taps based on a computed error signal; and
second adaptation circuitry in communication with said feedback filter circuitry, wherein said second adaptation circuitry is configured to adapt said second set of feedback taps based on said computed error signal.
18. The circuitry of
said first adaptation circuitry is further configured to perform a first least-mean-square algorithm and alter a value of a tap of said second set of feedforward taps based on a result of said performing said first least-mean-square algorithm; and
said second adaptation circuitry is further configured to perform a second least-mean-square algorithm and alter a value of a tap of said second set of feedback taps based on a result of said performing said second least-mean-square algorithm.
19. The circuitry of
decision device circuitry in communication with said feedforward filter circuitry and said feedback filter circuitry;
addition circuitry in communication with said decision device circuitry, wherein said addition circuitry is configured to compute said error signal based on an input signal of said decision device circuitry and an output signal of said decision device circuitry; and
sign circuitry in communication with said addition circuitry and said first and second adaptation circuitries, wherein said sign circuitry is configured to compute a sign of said computed error signal and to transmit said computed sign to said first and second adaptation circuitries.
20. The circuitry of
comparison circuitry in communication with said feedforward filter circuitry, wherein said comparison circuitry is configured to compare a magnitude of a lowest-magnitude tap of said second set of feedforward taps to a magnitude of a highest-magnitude tap in said first set of feedforward taps, wherein:
said magnitude of said lowest-magnitude tap is less than or substantially equal to a magnitude of any other tap of said second set of feedforward taps; and
said magnitude of said highest-magnitude tap is greater than or substantially equal to a magnitude of any other tap of said first set of feedforward taps;
feedforward tap removal circuitry in communication with said comparison circuitry, wherein said feedforward tap removal circuitry is configured to remove said lowest-magnitude tap from said second set of feedforward taps if said magnitude of said lowest-magnitude tap is less than said magnitude of said highest-magnitude tap; and
feedforward tap addition circuitry in communication with said comparison circuitry, wherein said feedforward tap addition circuitry is configured to add said highest-magnitude tap to said second set of feedforward taps if said magnitude of said lowest-magnitude tap is less than said magnitude of said highest-magnitude tap.
21. The circuitry of
tap sorter circuitry in communication with said feedforward filter circuitry, wherein said tap sorter circuitry is configured to sort said second set of feedforward taps by magnitude to form an ordered set of feedforward taps; and
tap selection circuitry in communication with said tap sorter circuitry, wherein said tap selection circuitry is configured to select at least one consecutive tap from said ordered set of feedforward taps, wherein each tap of said selected at least one consecutive tap has a magnitude that is greater than or substantially equal to a magnitude of any tap of said ordered set of feedforward taps that is not in said selected at least one consecutive tap.
22. The circuitry of
addition circuitry in communication with said feedforward filter circuitry and said feedback filter circuitry, wherein said addition circuitry is configured to generate an adder output signal; and
decision device circuitry in communication with said addition circuitry and said feedback filter circuitry, wherein said decision device circuitry is configured to compare a voltage of said adder output signal to at least one voltage threshold to generate a decision output signal.
23. The circuitry of
said feedforward filter circuitry is configured to perform at least one multiplication on said input signal and at least one value of said selected at least one tap; and
said feedback filter circuitry is configured to perform at least one multiplication on said decision output signal and at least one value of said second set of feedback taps.
24. A set-top box comprising the circuitry of
25. A high-definition television comprising the circuitry of
26. Circuitry for equalizing an input signal, said circuitry comprising:
multiplexer means for receiving a first set of feedforward taps with a plurality of data inputs, wherein respective ones of feedforward taps are coupled to respective ones of the plurality of data inputs;
feedforward filter means for receiving said input signal, wherein said feedforward filter means is coupled to an output of the multiplexer means;
feedback filter means for receiving a signal responsive to said input signal, wherein said feedback filter means is in communication with said feedforward filter means, and wherein said feedback filter means is configured to select a first set of feedback taps; and
control means for controlling feedforward taps of said multiplexer means and feedback taps of said feedback filter means, wherein said control means is in communication with said multiplexer means and said feedback filter means, said control means comprising:
selector means for generating a control signal received at a control input of the multiplexer, based on which one of the plurality of data inputs of the multiplexer is coupled to the output of the multiplexer to filter said input signal, wherein the coupling generates a second set of feedforward taps, wherein:
said selector means is in communication with said feedforward filter means; and
feedback tap addition means for adding a set of feedback taps to said first set of feedback taps to generate, in response to the coupling, a compensating second set of feedback taps to filter said input signal, wherein said feedback tap addition means is in communication with said feedback filter means.
27. The circuitry of
28. The circuitry of
means for computing said first set of feedback taps in accordance with a feedback Cholesky factorization; and
means for computing said first set of feedforward taps in accordance with a feedforward back-substitution.
29. The circuitry of
first adaptation means for adapting said second set of feedforward taps based on a computed error signal, wherein said first adaptation means is in communication with said feedforward filter means; and
second adaptation means for adapting said second set of feedback taps based on said computed error signal, wherein said second adaptation means is in communication with said feedback filter means.
30. The circuitry of
said first adaptation means comprises means for performing a first least-mean-square algorithm and altering a value of a tap of said second set of feedforward taps based on a result of said performing said first least-mean-square algorithm; and
said second adaptation means comprises means for performing a second least-mean-square algorithm and altering a value of a tap of said second set of feedback taps based on a result of said performing said second least-mean-square algorithm.
31. The circuitry of
decision device means for receiving a decision device input signal and generating a decision device output signal, wherein said decision device means is in communication with said feedforward filter means and said feedback filter means;
addition means for computing said error signal based on said decision device input signal and said decision device output signal, wherein said addition means is in communication with said decision device means; and
sign means for computing a sign of said computed error signal and transmitting said computed sign to said first and second adaptation circuitries, wherein said sign means is in communication with said addition means and said first and second adaptation circuitries.
32. The circuitry of
comparison means for comparing a magnitude of a lowest-magnitude tap of said second set of feedforward taps to a magnitude of a highest-magnitude tap in said first set of feedforward taps that is not in said second set of feedforward taps, wherein:
said comparison means is in communication with said feedforward filter means;
said magnitude of said lowest-magnitude tap is less than or substantially equal to a magnitude of any other tap of said second set of feedforward taps; and
said magnitude of said highest-magnitude tap is greater than or substantially equal to a magnitude of any other tap of said first set of feedforward taps that is not in said second set of feedforward taps;
feedforward tap removal means for removing said lowest-magnitude tap from said second set of feedforward taps if said magnitude of said lowest-magnitude tap is less than said magnitude of said highest-magnitude tap, wherein said feedforward tap removal means is in communication with said comparison means; and
feedforward tap addition means for adding said highest-magnitude tap to said second set of feedforward taps if said magnitude of said lowest-magnitude tap is less than said magnitude of said highest-magnitude tap, wherein said feedforward tap addition means is in communication with said comparison means.
33. The circuitry of
tap sorter means for sorting said second set of feedforward taps by magnitude to form an ordered set of feedforward taps, wherein said tap sorter means is in communication with said feedforward filter means; and
tap selection means for selecting at least one consecutive tap from said ordered set of feedforward taps, wherein:
each tap of said selected at least one consecutive tap has a magnitude that is greater than or substantially equal to a magnitude of any tap of said ordered set of feedforward taps that is not in said selected at least one consecutive tap; and
said tap selection means is in communication with said tap sorter means.
34. The circuitry of
addition means for generating an adder output signal, wherein said addition means is in communication with said feedforward filter means and said feedback filter means; and
decision device means for comparing a voltage of said adder output signal to at least one voltage threshold to generate a decision output signal, wherein said decision device means is in communication with said addition means and said feedback filter means.
35. The circuitry of
said feedforward filter means comprises means for performing at least one multiplication on said input signal and at least one value of said second set of feedforward taps; and
said feedback filter means comprises means for performing at least one multiplication on said decision output signal and at least one value of said second set of feedback taps.
36. A set-top box comprising the circuitry of
37. A high-definition television comprising the circuitry of
Description This application claims the benefit of provisional application No. 60/733,907, filed Nov. 4, 2005, which is hereby incorporated by reference herein in its entirety. This application relates to digital communication. More particularly, this application relates to equalizers used in digital communication to compensate for dispersive channels. A signal may be degraded when sent across a channel. A key challenge in communication theory is how to overcome this degradation to reliably and efficiently receive the correct series of elements that comprise a signal. One type of degradation is dispersion, where each element of a signal spreads in time, potentially overlapping with adjacent elements to create intersymbol interference (ISI). One example of dispersion occurs when an antenna sends a signal through the air. Because the signal is sent omnidirectionally, it can take multiple paths to reach the destination, with each path encountering its own set of obstacles. These obstacles can partially reflect the signal, partially absorb the signal, or both, resulting in the destination receiving multiple versions of the signal at varying strengths and at varying times. The main path usually yields the strongest version, with any preceding versions known as pre-echoes and any subsequent versions known as post-echoes. An equalizer is a device designed to compensate for signal dispersion. A common type of equalizer is the decision-feedback equalizer (DFE), which makes decisions based on pre-echoes to substantially cancel out post-echoes. The DFE is often capable of adapting to channels whose characteristics vary over time, yielding relatively accurate performance in the presence of significant ISI. For channels whose pre-echoes and post-echoes are spread over a relatively long period of time, known as long delay spread channels, the DFE often requires more computations to properly process the echoes to equalize the channel. More computations generally increase the amount of hardware and time required to achieve accurate performance. A need remains for an equalizer capable of more efficiently and reliably equalizing such channels. In accordance with this invention, methods and apparatus are provided for configuring equalization circuitry. In one aspect of the invention, at least one tap can be selected from a set of feedforward taps of feedforward filter circuitry. Each tap of the selected at least one tap has a magnitude greater than or substantially equal to a magnitude of any tap of the set of feedforward taps that is not in the selected at least one tap. At least one tap can be added to a set of feedback taps of feedback filter circuitry in communication with the feedforward filter circuitry. In another aspect of the invention, circuitry for equalizing an input signal can include feedforward filter circuitry operable to receive the input signal, feedback filter circuitry in communication with the feedforward filter circuitry, and control circuitry in communication with the feedforward and feedback filter circuitries. The control circuitry can include selector circuitry in communication with the feedforward filter circuitry and configured to select at least one tap from a set of feedforward taps of the feedforward filter circuitry, where each tap of the selected at least one tap has a magnitude that is greater than or substantially equal to a magnitude of any tap of the set of feedforward taps that is not in the selected at least one tap. The control circuitry can also include feedback tap addition circuitry in communication with said feedback filter circuitry and configured to add at least one tap to a set of feedback taps of the feedback filter circuitry. In yet another aspect of the invention, circuitry for equalizing an input signal can include feedforward filter means for receiving the input signal, feedback filter means for receiving a signal responsive to the input signal, and control means for controlling a set of feedforward taps of the feedforward filter means and a set of feedback taps of the feedback filter means. The feedback filter means can be in communication with the feedforward filter means and the control means can be in communication with the feedforward filter means and the feedback filter means. The control means can include selector means for selecting at least one tap from the set of feedforward taps, where the selector means is in communication with the feedforward filter means and each tap of the selected at least one tap has a magnitude that is greater than or substantially equal to a magnitude of any tap of the set of feedforward taps that is not in the selected at least one tap. The control means can further include feedback tap addition means for adding at least one tap to the set of feedback taps, where the feedback tap addition means is in communication with the feedback filter means. In yet another aspect of the invention, a computer program running on a processor can perform the steps of selecting at least one tap from a set of feedforward taps of feedforward filter circuitry, where each tap of the selected at least one tap has a magnitude greater than or substantially equal to a magnitude of any tap of the set of feedforward taps that is not in the selected at least one tap, and adding at least one tap to a set of feedback taps of feedback filter circuitry in communication with the feedforward filter circuitry. The computer program can also compute the set of feedback taps and compute the set of feedforward taps based on a result of the computing of the set of feedback taps. In one embodiment, the computing the set of feedback taps and the set of feedforward taps can be in accordance with, respectively, a feedback Cholesky factorization and a feedforward back-substitution. The computer program can also generate a feedforward output signal and a feedback output signal from, respectively, the feedforward filter circuitry and the feedback filter circuitry, and compute an error signal based at least in part on the feedforward and feedback output signals. The computer program can adapt the set of feedforward taps and the set of feedback taps based on the computed error signal. In one embodiment, the adapting the sets of taps can include altering a value of a tap of the set of feedforward taps and a value of a tap of the set of feedback taps based on a result of performing at least one LMS algorithm. In another embodiment, the computer program can compute a sign of the computed error signal. The computer program can also compare a magnitude of a lowest-magnitude tap of the selected at least one tap to a magnitude of a highest-magnitude tap in the set of feedforward taps that is not in the selected at least one tap. The magnitude of the lowest-magnitude tap can be less than or substantially equal to a magnitude of any other tap of the selected at least one tap, while the magnitude of the highest-magnitude tap can be greater than or substantially equal to a magnitude of any other tap of the set of feedforward taps that is not in the selected at least one tap. If the magnitude of the lowest-magnitude tap is less than the magnitude of the highest-magnitude tap, then the computer program can remove the lowest-magnitude tap from the selected at least one tap and add the highest-magnitude tap to the selected at least one tap. The computer program can also sort the set of feedforward taps by magnitude to form an ordered set of feedforward taps and select at least one consecutive tap from the ordered set of feedforward taps. Each tap of the selected at least one consecutive tap has a magnitude that is greater than or substantially equal to a magnitude of any tap of the ordered set of feedforward taps that is not in the selected at least one consecutive tap. The computer program can also: process an input signal using the set of feedforward taps to generate a feedforward output signal; add a signal responsive to the feedforward output signal to a signal responsive to a feedback output signal to generate an adder output signal; compare a voltage of a signal responsive to the adder output signal to at least one voltage threshold to generate a decision output signal; and process a signal responsive to the decision signal using the set of feedback taps to generate the feedback output signal. In one embodiment, the processing the input signal can include performing at least one floating-point (or fixed-point) multiplication on the input signal and at least one value of the selected at least one tap. The processing the signal responsive to the decision output signal can include performing at least one fixed-point (or floating-point) multiplication on the decision output signal and at least one value of the set of feedback taps. The computer program described above can be used to operate any suitable application, such as a high-definition television or a set-top box. The invention therefore advantageously allows more efficient and reliable equalization of channels. Advantageously, the invention allows such equalization with relatively accurate performance and relatively low complexity. The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts and in which: DFE Addition circuitry If DFE An equalizing first set of taps often has a number of elements proportional to the delay spread of the corresponding channel. More particularly, a channel with a long delay spread typically requires an equalizing first set of taps with a relatively large number of taps. A larger number of taps can require first adaptive filter circuitry Feedforward filter circuitry First addition circuitry Second addition circuitry First adaptation circuitry First adaptation circuitry First adaptation circuitry In one implementation, a LMS algorithm can update a vector v In accordance with an embodiment of the invention, control circuitry Control circuitry Control circuitry According to an embodiment of the invention, control circuitry An input delay line memory Multiplication circuitry Addition circuitry A set of significant taps can be selected from the initial set of feedforward taps at step The number of taps in the set of feedback taps can be adjusted at step Equalization can be maintained by adapting the set of feedforward taps and the set of feedback taps in response to changing channel characteristics at step The set of significant feedforward taps, the number of taps in the set of feedback taps, or both can be updated, if necessary, at step As the channel characteristics may continue to change throughout equalization, flow diagram Flow diagram Referring now to Referring now to The HDTV Referring now to The present invention may also be implemented in other control systems The powertrain control system Referring now to The cellular phone Referring now to The set top box Referring now to The media player Thus it is seen that methods and apparatus are provided for achieving efficient and reliable equalization of channels. One skilled in the art will appreciate that the invention can be practiced by embodiments other than those described, which are presented for the purposes of illustration and not of limitation, and the present invention is limited only by the claims which follow. Patent Citations
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